This invention relates to an open loop seeker aiming guiding system for directing a guided missile from launch to impact with a moving target. More particularly, this invention relates to a guiding system which combines the advantages of a command to line of sight guiding system with those of a fire and forget terminal homing/seeking guiding system.
Two embodiments of a command to line of sight guiding system are illustrated in FIGS. 1A and 1B and an embodiment of the fire and forget terminal homing/seeker guiding system is illustrated in FIG. 1C. All of FIGS. 1A, 1B, and 1C are prior art devices. To facilitate an understanding of the present invention these prior art devices will be described in some detail, hereinafter.
In FIG. 1A, the operator or the gunner 10 views target 16 through a device called the target tracker 18. The target tracker may simply be telescope with central cross-hairs to assist in accurate target aiming, a television camera and display, or a night device such as a thermal-imaging sight. In addition to the cross-hairs, it may have other aids to assist in accurate target tracking such as being mounted on a tripod or other stable mounting. It may also contain an inertial or viscous damping mechanism to smooth out tracking errors, or it may even include electronic image stabilization and/or tracking.
In the prior art device of FIG. 1A, the gunner's entire job during a missile operation is to acquire the target in his sight, to place the aiming cross-hairs over the target image, to fire the missile with a trigger pull, and then to keep the cross-hairs aligned over the target as smoothly as possible until the target is impacted by the missile.
Mounted as closely as possible to the target tracker 18 is a missile tracker 20. The missile tracker 20 is mechanically boresighted with the target tracker 18, so as to always possess a central tracking axis which is exactly parallel to the operator's line of sight to the target as established by the target tracker 18. The missile tracker 20 is also responsive to missile signature 36, which is usually augmented with an artificial signature-generating device 26 known as a missile beacon. If the missile is not centered on the missile tracker boresight axis, an error signal is generated which is representative of the missile angular offset from the missile tracker's boresight axis. This error signal is sent to guidance computer 22, which contains an angle-to-linear conversion device which multiples it by an approximation of the missile range, derived from the time expired following the launch of the missile. It also contains a low pass filtering device to reduce the noise effects, and lead circuits to compensate for measurement delays, to create a stable servo-mechanism with the missile dynamics when the control loop is closed. When the missile position voltages have been so modified they are called commands and are used to direct the missile back to the boresight axis of the target tracker.
The command signals are sent to the command transmitter 24, which may comprise a radio link, an optical link, or a wired link 38, to convey these commands to the missile in flight.
A missile receiver 30, which is located on the missile, and may comprise respectively a radio, or an optical or a telephonic receiver, accepts the commands and decodes them from a carrier (if they are so encoded) and sends the commands themselves to the missile autopilot 28. Autopilot 28 is an electronic/inertial package that is used to stabilize the missile guidance by measuring the body axis angles and using the angles to moderate the guiding commands. The guiding commands, so moderated, are sent to control mechanism 32, which may comprise vanes, thrusters or tail fins. The operation of control mechanism 32 causes the missile to maneuver vertically and/or horizontally toward the missile tracker boresight axis. The missile beacon, as a component mounted on the missile structure, is by this maneuver, re-positioned in space. The new position is then measured by the missile tracker 20 for a new computation, and the closed loop guiding system is completed.
This process continues, without knowledge of or use of range information, until the missile eventually impacts whatever object is under the cross-hairs of the gunner's target tracker 18.
Referring now to prior art FIG. 1B which shows a variation of a line of sight guidance system called the laser beamrider system. This variation comprises replacing the missile tracker 20 (in FIG. 1A) with beam projector 23 which projects a spatially coded beam 25 to a missile receiver 40. The beam projector 23 spatially modulates the beam over its cross-section and the modulation is received by the missile receiver 40. The missile position in space is determined by decoding this spatial information in the position decoder/corrector 42. The various low pass and lead filters, which were employed in the guidance computer 22 of FIG. 1A are also included in the decoder/corrector 42 of FIG. 1B. The output from the position decoder/corrector 42 to autopilot 28 is the same in the variation of FIG. 1B as it was in the system of FIG. 1A, and is used to control the flight of the missile in the same way as described in reference to FIG. 1A. A laser beam guiding system is described in detail in U.S. Pat. Nos. 3,782,667; 3,807,758; 4,696,411; and SIR No. H299.
Another prior art device is illustrated in FIG. 1C, which shows a general functional block diagram of an imaging seeker guidance system. The system illustrated in FIG. 1C functions in the following manner. An operator 10 first surveys his battlefield area of responsibility with a target acquisition device which is external to the missile system and is not shown. This device could be a radar system or as simple as a pair of binoculars. Upon detecting/selecting a suitable target, the gunner activates the missile system by applying power to at least the seeker and processing subsystems of missile 14. The fire control base 12 is usually located on the ground with the operator 10. At that point, the operator's display is automatically changed. A seeker image display 47 is shown in FIG. 1C, using outputs derived from 50, 52, 46 & 34, as described later.
The operator manually inserts target selection information into the imaging processing device 46, by the use of a light pen on the display screen or with a joystick input to place electronic cross hairs on the target. The image processor 46 then transmits a signal to a gimbal driver 48 to direct the seeker's gimbals, located on the missile, so that the target is in the center of the seeker field of view. In the event that the externally located target is not within the seeker field of view at all upon initial missile activation, operator 10 may directly input commands to the gimbal driver 48 to search an area through the seeker system. When the target is reacquired in the seeker image display 47, input target selection information is transmitted to the image processor, as noted before.
Once the image processor is correctly commanding the gimbal driver 48 so as to center the seeker's gimbals on the target, the missile may be fired by pulling the trigger. At that time, the fire control 12 is disconnected from the missile, which is launched and proceeds downrange towards the target. Operator 10 has no further interaction or control over the missile 14.
ln flight, the seeker receives target 16 signature signals through optics 50 and images them with imaging sensor 52. The imaging sensor 52 transmits its images through a signal output means 54 which selects, formats, and drives this data off-gimbal to the imaging processor 46. The imaging processor 46 tracks the preselected target 16 through the missile and target motions and generates correction signals which are sent to the gimbal driver 48. This causes the seeker boresight to remain centered on target 16, regardless of relative motion between the target 16 and missile 14.
A gimbal position sensor 56 measures the relative angle and angle rates between the seeker gimbals and the missile 14. These measurements are sent off-gimbal to the guiding algorithm computer 58, which may include an autopilot. For example, a simple pursuit navigation algorithm will simply fly the missile along the seeker line of sight to the target, i.e. turn the missile until the gimbal position sensor output is zero. A more accurate algorithm is a well known one called proportional navigation. This algorithm maneuvers the missile body until the angular rate from the gimbal position sensor 56 is zero independent of the actual angle. That will turn the missile in the direction to decrease the angle rate. When a near zero rate is achieved, the missile is on an interception path with moving target 16.
ln either case, the guidance algorithm computer 58 provides command signals to control mechanism 32, which causes the missile to maneuver until the algorithm is satisfied. This process continues until the missile eventually impacts with a target having the image being tracked by the image processor 46.